1. Field of the Invention
The present invention relates to electronic circuits, and more particularly relates to high speed electronic circuits propagating high frequency signals.
2. Art Background
Electronic circuits, and in particular computer and instrumentation circuits, have in recent years become increasingly powerful and fast. Driven in large part by customer demand, present day computer circuits are many times, and in cases, several orders of magnitude faster than their prior generation counterparts. When circuit frequencies and signal waveforms are sufficiently low, inductive reactance is small and only the resistance and capacitance of wires is significant. Electronic components may be represented as lumped circuit elements. However, as clock frequencies and the associated propagated signals exceed frequencies of many tens of MHz, inductive reactance increases and capacitance reactance decreases. The electrical wires are more sensitive to the surrounding geometry and structure and are modeled using transmission lines. For example, electrical signals operating at high frequencies, including clock and data signals, emit electric fields from their associated datapaths which couple to and affect neighboring signals. In fact, interconnections are becoming the limiting factor in how fast electronic circuits may operate, especially in VLSI (Very Large Scale Integration) and ULSI (Ultra Large Scale Integration) circuits.
In order to enhance the speed and performance of high speed electronic circuits, ground planes are used to improve the propagation of signals along electrical pathways. As suggested above in connection with the surrounding geometry of high frequency signal propagation, ground planes are desirable because they help control the impedance presented to a signal propagating along a wire, thereby reducing crosstalk and reflections. Reflections can be caused by variations in surface topography as a signal path traverses over steps, and other physical boundaries, or by impedance discontinuities along the signal path. Ground planes ensure that reflections in signal datapaths from variations in geometry are minimized, and that signal line impedance does not vary substantially as the signal path traverses the circuitry. Further, because the high frequency electric field emitting from a given signal path using a ground plane is concentrated between that signal path and the ground plane, crosstalk between the given signal path and an adjacent signal path is commensurately reduced. Ground planes are frequently incorporated into high frequency electronic circuitry because they are effective in reducing crosstalk and reflections in high frequency signal paths. For example, ground planes are common place in electronic circuits operating at RF and microwave frequencies.
Although known in the prior art, fabrication of ground planes in electronic circuits remains cumbersome, requiring two distinct material deposition steps and two distinct patterning operations. In particular, a ground plane metal would typically be deposited upon a previously deposited dielectric layer, whereafter the ground plane metal is patterned and etched. Thereafter, a second dielectric layer is deposited, patterned, and etched in an appropriate fashion to insulate the ground plane metal. In order to make electrical contact with external control and data signals, metallic conductors that are below the ground plane metal must rise up and pass through the ground plane metallization without making physical contact to it. Where such underlying metallic conductors must pass through the ground plane, it is necessary to electrically insulate the conductor metal from the ground plane metal to prevent shorting. Accordingly, the "double deposition" and "double patterning" methods have been exclusively used in the prior art to insulate the metallic conductor passing through the ground plane metal. Patterning operations are complex in that they typically require a photolithographic process and etching process. The added complexity will have an effect on the yield of the product and in turn, the cost.
As will be explained in the following detailed description, the present invention discloses a new ground plane and sidewall insulator structure which may be used to singly or in combination to produce embedded ground planes or, alternatively, shielded conductor signal paths. Moreover, the present invention provides methods requiring fewer processing operations for producing an embedded ground plane using sidewall insulation for interconnections passing through the ground plane.